The impact of additive and oil chemistry on low speed pre-ignition (LSPI) was evaluated. An additive metals matrix varied the levels of zinc dialkyldithiophosphate (ZDDP), calcium sulfonate, and molybdenum within the range of commercially available engine lubricants. A separate test matrix varied the detergent chemistry (calcium vs. magnesium), lubricant volatility, and base stock chemistry. All lubricants were evaluated on a LSPI test cycle developed by Southwest Research Institute within its Pre-Ignition Prevention Program (P3) using a GM LHU 2.0 L turbocharged GDI engine. It was observed that increasing the concentration of calcium leads to an increase in the LSPI rate. At low calcium levels, near-zero LSPI rates were observed. The addition of zinc and molybdenum additives had a negative effect on the LSPI rate; however, this was only seen at higher calcium concentrations. Displacing some or all of the calcium with magnesium reduces the LSPI rate relative to an all-calcium lubricant. There was a minor impact of volatility, but the statistical analysis concluded it was insignificant. The impact of viscosity was significant with lower LPSI rates observed with the low viscosity oil at high magnesium concentrations.
It is clear that even with the relatively simple formulation changes studied in these test matrices, the LSPI rate of the engine can be significantly impacted. It can be expected that other common oil additive chemistries would also impact the LSPI rate based on these results. Given these results, there is no indication that the general trend towards lower viscosities will prove problematic for LSPI. There is also the potential for improving LSPI rates without reducing detergent concentrations if the observed magnesium result can be confirmed. In the long term, it will still be critical to develop a fundamental understanding of the chemistry which makes the detergent an active part of the LSPI process.
